I have this doubt from the time I started reading Hindu Purana. In Hindu Purana there are many stories about interaction between Asuras, Monks, Devas and common people. According to my knowledge Mahabali was a king of Asuras , who ruled the people. I want know that what happened to the Asuras? Do they still exist? If so, why don't they interact with people?
Well they are still existing! The Devi Bhagwat Puran Book 8 Chapter 19 gives the following account of the lower lokas that are the residences of various Asura species. Talking about the region of Vitala it states:
Vitala is situated below the earth. There the Bhagavân Bhava, worshipped by all the Devas, has assumed the name of Hâtakes’vara and is staying there coupled with Bhavânî, surrounded by His attendants specially for the increase of the creation of Brahmâ. The river Hâtaki flows there and has Her origin from the essences of them both. Fire, augmented by the help of the wind, begins to drink it. When the Fire leaves that, making a Phutkâra noise, the gold, named Hâtaka, is created. This gold is very much liked by the Daityas. The Daitya women use this gold always for their ornaments.
It further mentions the loka called Sutala where the leader of Asurs - Maharaj Bali resides.
Below Vitala is Sutala. It is reckoned as of some special importance. O Muni! The highly meritorious Bali, the son of Virochana lives here. The Bhagavân Vâsudeva, brought down this Bali into Sutala, for the welfare of Indra. He assumed the body of Trivikrama and gave to Bali all the wealth of the three Lokas, all the Laksmî went to him and installed him in the position of the Lord of the Daityas. What more can be said than this, that what prosperity, wealth and riches that Indra could not obtain, that S’rî Laksmî Devî Herself has followed Bali. Bali, as the Lord of Sutala, has become entirely fearless, remains here upto this day and is worshipping Vâsudeva.**
Since Maharaj Bali was blessed by Lord Vishnu to become the next Indra, he is definitely alive. He is also amongst one of the eight Chiranjivis or Immortals in Hindu mythology.
Chapter 20 of the Devi Bhagwat continues with the narrative mentioning other Asurs:
1-37. Nârâyana said :-- O Nârada! The cave lower down than Sutala is Talâtal! The Lord of Tripura, (the three cities) the great Mâyâ Dânava is the Ruler of this region. Mahes’vara, the Doer of good to the three Bhuvanas, burnt his three cities; but at last, being pleased with his devotion, He rescued him. Thus Mâyâ, by the favour of that God, has regained his own kingdom and the enjoyments thereof. This Mâyâ Dânava is the Teacher (Âchârya) of the Mâyâvi sect and the cult thereof; and he is skilled in various Mâyâs or all sorts of the magic powers. All the fierce demons, of cruel temper, worship him for their prosperities in their various enterprises.
Below the Talâtala is the loka called Mahâtal where the serpents reside so we'll skip the description and move on to the next one Rasâtal.
The Daityas, Dânavas and Pani Asuras live here. Besides these, there live the Nivâta Kavachas of the Hiranyapura city and the Asuras named Kâleyas, the enemies of the Devas. These all are naturally very energetic and brave; their powers are baffled by the Tejas of the Bhagavân and they live like snakes in this region. The other Asuras that were driven and were afraid of the Mantras, uttered by Saramâ, the messenger of Indra, live here too.
Thus the demons are very much there thriving in their own lokas.
Coming to the second part of your question
The reason they do not appear to interact with us maybe two fold:
Their life spans are much longer than ours so we may just about see a glimpse of one of them sometime if lucky (or unlucky depending on how you take it).
They may be around us and we may not even recognize them! For example, in the Mahabharat times also people didn't realize that Kauravs, Kamsa, Jarasandh etc. were actually Asuras as they appeared as humans. SECTION LXVII of Sambhava Parva lists all the incarnations of gods as well as demons and no one even knew their truth most of the time!
The first of Danavas, who was known by the name of Viprachitti, became that bull among men, noted as Jarasandha. And, O king, that son of Diti, who was known as Hiranyakasipu, was known in this world among men as the powerful Sisupala. He who had been known as Samhlada, the younger brother of Prahlada, became among men the famous Salya, that bull amongst Valhikas. The spirited Anuhlada who had been the youngest became noted in the world as Dhrishtaketu. And, O king, that son of Diti who had been known as Sivi became on earth the famous monarch Druma. And he who was known as the great Asura Vashkala became on earth the great Bhagadatta. The five great Asuras gifted with great energy, Ayahsira, Aswasira, the spirited Aysanku, Gaganamurdhan, and Vegavat, were all born in the royal line of Kekaya and all became great monarchs. That other Asura of mighty energy who was known by the name of Ketumat became on earth the monarch Amitaujas of terrible deeds. That great Asura who was known as Swarbhanu became on earth the monarch Ugrasena of fierce deeds. That great Asura who was known as Aswa became on earth the monarch Asoka of exceeding energy and invincible in battle. And, O king, the younger brother of Aswa who was known as Aswapati, a son of Diti, became on earth the mighty monarch Hardikya. The great and fortunate Asura who was known as Vrishaparvan became noted on earth as king Dirghaprajna. And, O king, the younger brother of Vrishaparvan who was known by the name of Ajaka became noted on earth as king Salwa. The powerful and mighty Asura who was known as Aswagriva became noted on earth as king Rochamana. And, O king, the Asura who was known as Sukshma, endued with great intelligence and whose achievements also were great, became on earth the famous king Vrihadratha. And that first of Asuras who was known by the name of Tuhunda, became noted on earth as the monarch, Senavindu. That Asura of great strength who was known as Ishupa became the monarch Nagnajita of famous prowess. The great Asura who was known as Ekachakra became noted on earth as Pritivindhya. The great Asura Virupaksha capable of displaying various modes of fight became noted on earth as king Chitravarman.
The first of Danavas, the heroic Hara, who humbled the pride of all foes became on earth the famous and fortunate Suvahu. The Asura Suhtra of great energy and the destroyer of foemen, became noted on earth as the fortunate monarch, Munjakesa. That Asura of great intelligence called Nikumbha, who was never vanquished in battle was born on earth as king Devadhipa, the first among monarchs. That great Asura known amongst the sons of Diti by the name of Sarabha became on earth the royal sage called Paurava. And, O king, the great Asura of exceeding energy, the fortunate Kupatha, was born on earth as the famous monarch Suparswa. The great Asura, O king, who was called Kratha, was born on earth as the royal sage Parvateya of form resplendent like a golden mountain. He amongst the Asura who was known as Salabha the second, became on earth the monarch Prahlada in the country of the Valhikas. The foremost, among the sons of Diti known by the name of Chandra and handsome as the lord of the stars himself, became on earth noted as Chandravarman, the king of the Kamvojas. That bull amongst the Danavas who was known by the name of Arka became on earth, O king, the royal sage Rishika. That best of Asuras who was known as Mritapa became on earth, O best of kings, the monarch, Pascimanupaka. That great Asura of surpassing energy known as Garishtha became noted on earth as king Drumasena. The great Asura who was known as Mayura became noted on earth as the monarch Viswa. He who was the younger brother of Mayura and called Suparna became noted on earth as the monarch, Kalakirti. The mighty Asura who was known as Chandrahantri became on earth the royal sage Sunaka. The great Asura who was called Chandravinasana became noted on earth as the monarch, Janaki. That bull amongst the Danavas, O prince of the Kuru race, who was called Dhirghajihva, became noted on earth as Kasiraja. The Graha who was brought forth by Sinhika and who persecuted the Sun and the Moon became noted on earth as the monarch Kratha. The eldest of the four sons of Danayu, who was known by the name of Vikshara, became known on earth the spirited monarch, Vasumitra. The second brother of Vikshara, the great Asura, was born on earth as the king of the country, called Pandya. That best of Asuras who was known by the name of Valina became on earth the monarch *Paundramatsyaka**. And, O king, that great Asura who was known as Vritra became on earth the royal sage known by the name of Manimat. That Asura who was the younger brother of Vritra and known as Krodhahantri became noted on earth as king Danda. That other Asura who was known by the name Krodhavardhana became noted on earth as the monarch, Dandadhara. The eight sons of the Kaleyas that were born on earth all became great kings endued with the prowess of tigers. The eldest of them all became king Jayatsena in Magadha. The second of them, in prowess, like Indra, became noted on earth as Aparajita. The third of them, endued with great energy and power of producing deception, was born on earth as the king of the Nishadas gifted with great prowess. That other amongst them who was known as the fourth was noted on earth as Srenimat, that best of royal sages. That great Asura amongst them who was the fifth, became noted on earth as king Mahanjas, the oppressor of enemies. That great Asura possessing great intelligence who was the sixth of them became noted on earth as Abhiru, that best of royal sages. The seventh of them became known throughout earth, from the centre to the sea, as king Samudrasena well acquainted with the truths of the scriptures. The eighth of the Kaleyas known as Vrihat became on earth a virtuous king ever engaged in the good of all creatures. The mighty Danava known by the name of Kukshi became on earth as Parvatiya from his brightness as of a golden mountain. The mighty Asura Krathana gifted with great energy became noted on earth as the monarch Suryaksha. The great Asura of handsome features known by the name of Surya, became on earth the monarch of the Valhikas by name Darada, that foremost of all kings. And, O king, from the tribe of Asuras called Krodhavasa, of whom I have already spoken to thee, were born many heroic kings on earth. Madraka, and Karnaveshta, Siddhartha, and also Kitaka; Suvira, and Suvahu, and Mahavira, and also Valhika, Kratha, Vichitra, Suratha, and the handsome king Nila; and Chiravasa, and Bhumipala; and Dantavakra, and he who was called Durjaya; that tiger amongst kings named Rukmi; and king Janamejaya, Ashada, and Vayuvega, and also Bhuritejas; Ekalavya, and Sumitra, Vatadhana, and also Gomukha; the tribe of kings called the Karushakas, and also Khemadhurti; Srutayu, and Udvaha, and also Vrihatsena; Kshema, Ugratirtha, the king of the Kalingas; and Matimat, and he was known as king Iswara; these first of kings were all born of the Asura class called Krodhavasa. There was also born on earth a mighty Asura known amongst the Danavas by the name of Kalanemi, endued with great strength, of grand achievements, and blessed with a large share of prosperity. He became the mighty son of Ugrasena and was known on earth by the name of Kansa.
I find the second option intriguing and for all we know the terrorists of today may actually be the Asuras in disguise!